There are more black holes in the early Universe than expected, according to observations made by the Hubble Space Telescope.
Because observatories like Hubble and the James Webb Space Telescope are so powerful, they can peer far into the cosmos, capturing light that’s been travelling for billions of years and therefore looking back in time.
This gives astronomers views of the early Universe, enabling them to see what it looked like shortly after the Big Bang.
By doing so, astronomers can learn more about the scale and structure of galaxies and, in this case, black holes, when the cosmos was in its infancy.
An international team of astronomers led by scientists in the Department of Astronomy at Stockholm University have used this technique and discovered more black holes in the early Universe than previously reported.
A key aspect of the study is in understanding more about supermassive black holes, which lie at the centre of many galaxies.
These central supermassive black holes are thought to play a key role in galaxies’ formation and evolution, and are key to a chicken-and-egg mystery: do galaxies for around supermassive black holes, or do supermassive black holes form within galaxies?
Scientists don’t currently have full knowledge of how the first black holes formed shortly after the Big Bang.
Early supermassive black holes that exist at the centre of galaxies less than a billion years after the Big Bang can weigh more than a billion Suns.
“Many of these objects seem to be more massive than we originally thought they could be at such early times — either they formed very massive or they grew extremely quickly,” says Alice Young, a PhD student from Stockholm University and co-author of the study published in The Astrophysical Journal Letters.
How Hubble saw the early black holes
In order to understand more about galaxy and black hole evolution, the team used the Hubble Space Telescope to observe how many black holes exist among faint galaxies in the very early Universe.
The first deep imaging of the Hubble Ultra Deep Field was carried out in 2004, then again years later, and again in 2023.
By comparing Hubble near-infrared exposures in 2009, 2012 and 2023, the team found evidence for flickering supermassive black holes in the hearts of early galaxies.
This meant the team could measure variations in the brightness of the galaxies: a tell-tale sign of black holes.
And the team identified more black holes than previously found by other methods.
This suggests some black holes may have formed by the collapse of massive stars during the first billion years of the Universe.
These young, pristine stars can only exist in the very early Universe because later generations are polluted by the remnants of stars that have already lived and died.
The team say other alternatives for black hole formation include collapsing gas clouds, stars in clusters colliding and merging, and so-called ‘primordial’ black holes that may have formed mere seconds after the Big Bang.
“The formation mechanism of early black holes is an important part of the puzzle of galaxy evolution,” says Matthew Hayes from the Department of Astronomy at Stockholm University and lead author of the study.
“Together with models for how black holes grow, galaxy evolution calculations can now be placed on a more physically motivated footing, with an accurate scheme for how black holes came into existence from collapsing massive stars.”
Read the full study in the Astrophysical Journal Letters at iopscience.iop.org/article/10.3847/2041-8213/ad63a7